Gas flow control in a ventilator

PRIORITY CLAIM

p-0002This application claims priority from Swedish Patent Application Ser. No. 0402120-0 filed Sep. 3, 2004.

TECHNICAL FIELD

p-0003The present invention pertains to a ventilator adapted to deliver a flow of gas to an airway of a user, and, in particular, to a ventilator with improved pressure control during inspiration.

BACKGROUND OF THE INVENTION

p-0004It is well known to utilize a ventilator, anesthesia machine, or pressure support device to deliver a fluid, such as oxygen, air, or other breathing gas or gas mixture, to an airway of patient to augment, supplement, or substitute the patient's own ventilatory effort and/or to treat the patient with a pressure support therapy. Of importance in such situations is the ability to accurately regulate or control the pressure, flow, and/or volume of gas delivered to the patient during the inspiratory phase of the respiratory cycle. For present purposes, the term “ventilator” is used to describe any system or device that delivers a flow of gas or pressurized gas to the airway of a user.

p-0005<figref idrefs="DRAWINGS">FIG. 1</figref> illustrates the inspiratory components of a conventional ventilator. These components include a source of a first gas <b>30</b>, such as air, and a source of a second gas <b>32</b>, such as oxygen. The source of first gas typically includes a pressurized storage tank, blower, bellows, impeller, fan, piston, pressure generator, or the like, that provides pressured air at a pressure above ambient pressure. The source of oxygen is typically a pressurized oxygen storage tank, a central wall supply (typically found in a hospital), or an oxygen concentrator. In short, the sources of the first and second gas can be pressure generators that operate under the control of the ventilator, an independent gas supply, such as that available through a hospital's central gas delivery system, or a combination thereof.

p-0006In the embodiment of <figref idrefs="DRAWINGS">FIG. 1</figref>, a first valve <b>34</b> controls the mixing of the first gas and the second gas, and a second valve <b>36</b> control the pressure and/or flow of the gas mixture provided to the patient, as indicated by arrow A. Valves <b>34</b> and <b>36</b> are typically proportional valves, which are commercially available in a number of different variants. Such valves usually comprise an electromagnet, a membrane that usually is made of rubber, and a valve seat. The amount or percentage that the valve is opened is determined by the current flowing through the electromagnet. The resulting gas flow is approximately proportional to the current.

p-0007An alternative conventional inspiratory portion of a ventilator is shown in <figref idrefs="DRAWINGS">FIG. 2</figref>. In this embodiment, separate valves <b>38</b> and <b>40</b> control the supply of gas to the first gas (e.g., air) and the supply of the second gas (e.g., oxygen). The separate gas supplies are mixed downstream of the valves, typically using a mixing element or accumulator, for subsequent delivery to the patient. In each embodiment, the combined gas flow is carried by a primary conduit <b>42</b> to an external coupling provided on the ventilator housing. A flexible hose or patient circuit (not shown) couples to the external coupling an airway of the patient.

p-0008A conventional ventilator typically includes a flow sensor <b>44</b> and a pressure sensor <b>46</b> to measure the flow and pressure, respectively, of the gas delivered to the patient via the patient circuit. The output of flow sensor <b>44</b> and pressure sensor <b>46</b> is provided to a controller <b>50</b>, which, in some ventilation modes, uses this information to control the flow, volume, and/or pressure of gas delivered to the patient. For example, the processor uses this information to control the actuation of valves <b>34</b>, <b>36</b>, <b>38</b>, or <b>40</b> so that the desired flow, pressure, or volume of gas is administered to the patient.

p-0009In order to control the flow, volume, or pressure of gas delivered to the patient, the flow sensor and/or pressure sensors are used in a “closed loop” or feedback configuration. That is, the signals output by these sensors are provided to controller <b>50</b>, which uses them to compare the actual flow, pressure, or volume, as determined from the sensors, with a set or desired quantity. The controller then controls the valves to reduce the error between the measured and the desired values. A typically controller <b>50</b> includes a PI or PID controller for comparing the measured values to the desired values and controlling the valves based thereon. An example of conventional ventilators that use such control techniques to provide pressure or volume control are described in U.S. Pat. Nos. 5,400,777 and 5,265,594. One disadvantage of conventional pressure/flow/volume control techniques, which is discussed in greater detail below, is that it is difficult for provide a control system that can quickly and accurately regulate the pressure, flow, or volume of gas using such techniques with a high level of stability.

DISCLOSURE OF THE INVENTION

p-0010Accordingly, it is an object of the present invention to provide a ventilator that overcomes the shortcomings of conventional ventilators. This object is achieved according to one embodiment of the present invention by providing a ventilator that includes a source of gas, a conduit that carries the flow of gas to the airway of a patient, a first valve that controls a pressure, flow, or volume, of the flow of gas, a pressure sensor operatively coupled to the conduit between the valve and the patient and adapted to monitor a pressure of the flow of gas in the conduit, and a controller adapted to control the valve based on an output of the pressure sensor. The ventilator further includes a restrictor provided in the conduit between the pressure sensor and the patient. The restrictor, in essence, divides the fluid delivery system such that a first volume is defined in the conduit between the valve and the restrictor, and a second volume is defined in the conduit between the patient and the restrictor and which includes the volume of the patient, wherein the first volume is less than the second volume. The control system controls the pressure, flow, or volume relative to the first volume so that an accurate, fast, and stable control of the pressure, flow, or volume is achieved

p-0011These and other objects, features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0012<figref idrefs="DRAWINGS">FIGS. 1 and 2</figref> are schematic diagrams of two examples of conventional ventilators;

p-0013<figref idrefs="DRAWINGS">FIG. 3</figref> is a schematic diagram of a first embodiment of a ventilator according to the principles of the present invention;

p-0014<figref idrefs="DRAWINGS">FIG. 4</figref> is a schematic diagram of a second embodiment of a ventilator according to the principles of the present invention;

p-0015<figref idrefs="DRAWINGS">FIG. 5</figref> is a schematic diagram illustrating the control system in a conventional ventilator;

p-0016<figref idrefs="DRAWINGS">FIG. 6</figref> is a schematic diagram illustrating the control system in a ventilator according to the principles of the present invention;

p-0017<figref idrefs="DRAWINGS">FIG. 7</figref> is a perspective view of a valve assembly for use in the ventilator of <figref idrefs="DRAWINGS">FIG. 4</figref>;

p-0018<figref idrefs="DRAWINGS">FIG. 8</figref> is an exploded view of the valve assembly of <figref idrefs="DRAWINGS">FIG. 7</figref>;

p-0019<figref idrefs="DRAWINGS">FIG. 9</figref> is a cross-sectional view of the valve assembly of <figref idrefs="DRAWINGS">FIG. 7</figref>;

p-0020<figref idrefs="DRAWINGS">FIG. 10A-10C</figref> illustrate a first embodiment of a schematic circuit diagram for controlling the operation of the valve assembly using pulse modulation; and

p-0021<figref idrefs="DRAWINGS">FIG. 11</figref> illustrates a second embodiment of a schematic circuit diagrams for controlling the operation of the valve assembly using pulse modulation.

BEST MODES FOR CARRYING OUT THE INVENTION

p-0022<figref idrefs="DRAWINGS">FIG. 3</figref> is a schematic diagram of a first embodiment of a ventilator <b>60</b> according to the principles of the present invention. It should be noted that <figref idrefs="DRAWINGS">FIG. 3</figref> illustrates only those components of the ventilator associated with the delivery of a flow of gas to a patient during the inspiratory phase of the respiratory cycle. The expiratory components are omitted. It should be further understood that the present invention contemplates that ventilator <b>60</b> can be a pressure support system, such as a CPAP or bi-level system. Such systems are a single limb systems. Instead of an expiratory limb, an exhaust vent is provided at or near the patient in the inspiratory limb. Such systems do not include expiratory related components.

p-0023It can be appreciated from reviewing <figref idrefs="DRAWINGS">FIGS. 1 and 3</figref>, that the ventilator of the present invention is similar in many respects to a conventional ventilator. The main difference is the ventilator <b>60</b> includes a flow restrictor <b>62</b> provided in conduit <b>42</b> downstream of pressure/flow control valve <b>36</b>. Thus, a source of a first gas <b>30</b> includes a pressurized storage tank, blower, bellows, impeller, fan, piston, pressure generator, compressors or any device that is capable of delivering gas at a pressure above ambient pressure. The source of first gas can be integral with the ventilator, i.e., disposed in the ventilator housing, which is typical if the source of first gas is a bellows, piston, compressor, blower, or the like. The present invention also contemplates that the source of first gas can be external to the ventilator, which is typical if the source of the first gas is a wall supply in a hospital. If the source of the first gas is external to the ventilator, input couples are provided on the ventilator that communicate the flow of gas to the ventilator. The source of second gas <b>32</b>, such as oxygen, is typically a pressurized oxygen storage tank, a wall supply, or an oxygen concentrator, or any device capable of providing a supplemental gas.

p-0024The present invention contemplates that the ventilator of the present invention can include devices, components, software, communication links, etc., typically associated with ventilators. Examples of devices typically used with a ventilator include humidifiers, nebulizers, filters, etc. Although not shown, a user interface device can be provided to allow a user to manually set up and/or control the ventilator. This interface can be provided directly on the ventilator in the form of a keypad, touchscreen, knob, dials, etc., or it can be remote therefrom with a hardwired or wireless communication link being used to communicate the remote device with the ventilator to set up and/or control the ventilator.

p-0025In order to perhaps best understand the function of restrictor <b>62</b>, the pressure/volume/flow control used in a conventional ventilation system must first be explained. As shown in <figref idrefs="DRAWINGS">FIG. 5</figref>, downstream of pressure/flow control valve <b>36</b> there exists a relatively large physical volume, which is represented by volume V_A. This volume includes the volume within the conduits that are internal to the ventilator, the flexible patient circuit coupled to the external coupling on the ventilator, the patient interface device that communicated the patient circuit with the patient's airway, the airways of the patient, which including the mouth, upper airway, trachea, and lungs. In a conventional ventilator, this relatively large volume is monitored by pressure sensor <b>46</b>. The control system, which includes controller <b>50</b> and valve <b>36</b> attempts to control the system accurately based on this monitored parameter. For example, if the patient it to receive a flow of gas at a target pressure of 10 cmH₂O, the pressure sensor will measure the actual pressure and the controller will adjust the valve in an attempt to deliver the flow of gas at that target pressure.

p-0026However, effective pressure regulation is difficult in this type of arrangement due to the fact that the control system is attempting to regulate a relatively large volume that has inherent instability. For example, the ability to control the pressure deteriorates as a due to the transit time that it takes for a pressure change to effect a large volume of fluid: the greater the volume of fluid, the longer the transmit time. In other words, the large the volume of fluid being controlled by the control system, the slower the system responds to pressure changes. In addition, the patient circuit (tubes) and the patient himself or herself have internal resistances and volumes that affect the ability of a pressure change induced by the valve to take effect in the whole system. The tubes and the patient's respiratory system also include a certain amount of inherent flexibility, which is referred to as elastance, so that pressure changes cause the volume to expand or contract, thereby changing the volume on the control system. It can be appreciated that changes in the volume as the pressure is increased or decreased by the controller make it harder for the control system to accurately control that volume to a certain pressure level. In addition, the fluid itself is compressible. This effectively results in low pass filtering of the pressure generation between the valve and the pressure transducer. Thus, the control system has difficult taking the measurements for pressure transducer <b>46</b> and using these signal to accurately and quickly control the pressure in a stable manner.

p-0027Restrictor <b>62</b>, in essence, separates volume V_Ainto two smaller volumes V_Band V_C, as illustrated in <figref idrefs="DRAWINGS">FIG. 6</figref>. Pressure sensor <b>46</b> is arranged so as to measure the pressure of the smaller and closer to the valve <b>36</b> volume V_B, and the control system (including controller <b>50</b> and valve <b>36</b>) is arranged to control the pressure of smaller volume V_B. In an exemplary embodiment of the present invention, volume V_Cis at least two times that of V_B. However, the present invention contemplates that volume V_Ccan be ten times that of V_Bor more. The control system controls the pressure of volume V_B. Because this is a relatively small volume, with less inherent instability, e.g., less resistance to flow, less internal elastance, less gas mass to accelerate, and a relatively small volume of fluid contained therein, the control system is better able to accurately and quickly control the pressure of volume V_B, and in a much more stable fashion, than that of volume V_A. Thus, restrictor <b>62</b> partly isolates the pressure control loop from the large volumes to make it easier for the control system to control the pressure of the smaller volume accurately.

p-0028There is a fluid communication between volumes V_Band V_Cthrough the restrictor. Thus, pressure control that is done with respect to volume V_Bis translated into volume V_C, albeit with a slight lag or delay. However, those skilled in the are can appreciated that the size of the restriction can be selected so as to minimize this delay will still providing a useful amount of separation between volumes V_Band V_Cso that the control system functions with the desired degree of precision. It should also be noted that restrictor <b>62</b> is also configured such that the gas flow through the restriction is sufficient to provide an adequate ventilation of the patient with a certain pre-determined gas volume, e.g., a maximum minute volume. In an exemplary embodiment of the present invention, the flow restriction created by restrictor <b>62</b> is at least two times greater than a resistance along the conduit from valve <b>36</b> to the patient.

p-0029It should be further noted that providing an intentional restriction in the gas flow downstream of the pressure/flow controlling valves is counterintuitive to the proper function of a conventional ventilator. Using conventional wisdom, the flow of gas existing the valve is provided to the patient with as little resistance as possible so that there only is a minimal amount of pressure drop between the pressure controller (the valve) and the patient. Flow restrictions downstream of the valves in a conventional ventilator adversely affect their ability to properly deliver a flow of gas to the patient.

p-0030In an exemplary embodiment of the present invention, restrictor <b>62</b> is configured to have a pre-determined pressure-flow curve. This curve may have any one of a variety of configurations, for example linear or non-linear. A variety of configurations for restrictor <b>62</b> are contemplated by the present invention to provide the desired pressure-flow curve. For example, the restrictor may take the form of a fixed element, such as a net, mesh, screen, aerodynamically shaped element or elements, disposed in the flow path of conduit <b>42</b>. The restrictor may also include one or more movable elements, such as flaps, slats, vanes, that alter the opening or geometry of the restrictor with changes in the pressure or flow to which the restrictor is subject. The present invention further contemplates that restrictor <b>62</b> can be removably disposed in conduit <b>42</b> so that different sizes, shapes, or configurations of restrictors can be chosen for different patient categories. This also allows for ease of cleaning of the restrictor. Moreover, the restrictor can be configured so as to have an adjustable flow restriction, so that the degree of flow restriction can be controlled either manually or automatically. For example the present invention contemplates that controller <b>50</b> may adjust the amount restriction provided by restrictor based on the monitored conditions of the system and/or patient, so that a suitable degree of restriction is provided that strikes a balance between the objectives noted above are achieved.

p-0031In a further exemplary embodiment, the restriction provided by restrictor <b>62</b> is variable so that the amount of fluid communication between volumes V_Band V_Ccan be dynamically adjusted, e.g., for low minute volumes, the restriction is adjusted to a maximum value, so that the control of the pressure in volume V_Cis as fast as possible. This is particularly advantageous in the ventilation of children or neonates, in which case it should be possible to control small minute volumes with high accuracy and with quickness of the pressure control. Likewise, where large minute volumes are needed, the restriction may also be released, such that still a restriction is present, but in such a way that higher instantaneous flows may be generated by the inspiratory portion of the ventilator. As noted above the variable choking function of the restrictor may be implemented in different ways, e.g., elements such as nets or aerodynamically shaped elements, can be moveably inserted into the flow channel.

p-0032In the embodiment illustrated in <figref idrefs="DRAWINGS">FIG. 3</figref>, volume V_Bcorresponds to the volume of the tubes, conduits, and other pneumatic components between valve <b>34</b> and restrictor <b>62</b>. The present invention also contemplates providing an additional volume, for example, a chamber coupled to conduit <b>42</b> between valve <b>34</b> and restrictor <b>62</b>, so that volume V_Bis sufficient for pressure control purposes.

p-0033The present invention also contemplates that volume V_Bcan be a variable volume. For example, a piston, collapsible tube, or any other mechanism for selectively changing a volume, can be coupled to volume V_B(e.g., by coupling the adjustable volume to conduit <b>42</b> between valve <b>34</b> and restrictor <b>62</b>, so that volume V_Bis adjustable and, hence, controllable. This is useful, for example, in situations where it is desirable to maintain a certain ratio between volume V_Band volume V_C.

p-0034A second embodiment of a ventilator <b>30</b>′ according to the principles of the present invention is schematically illustrated in <figref idrefs="DRAWINGS">FIG. 4</figref>. In the embodiment, like that of the conventional ventilator shown in <figref idrefs="DRAWINGS">FIG. 2</figref>, two valves <b>38</b> and <b>40</b> are provided to control the flow of a first gas from first source <b>30</b> via gas flow path <b>63</b> and a second gas from second source <b>32</b> via gas flow path <b>65</b>. The two gas flows, which are illustrated by arrows B and C are combined to define gas flow A for delivery to the user. In this embodiment, flow sensors <b>64</b> and <b>66</b> are disposed in the gas flow paths to monitor the direction and/or rate of flow of each gas prior to being mixed. Restrictor <b>62</b>, as discussed above, is provided downstream of valves <b>64</b> and <b>66</b>, and a pressure sensor is coupled to the gas flow path between valves <b>64</b> and <b>66</b> and restrictor <b>62</b>.

p-0035Although not illustrated in this figure, the present invention contemplates that other sensors can be provided in the ventilatory system and used on fine-tune the control the system. For example, pressure sensors can be provided along the patient circuit to monitor the pressure at a location along the circuit. This is helpful in order to characterize dynamical pressure drops and in order to compensate the pressure regulator for the purpose of achieving a more accurate pressure regulation. The present invention, also contemplates providing a pressure sensor at, near, or in the patient's airway to provide an accurate measure of the actual pressure experienced by the patient. Again, this pressure can be used to provide even better control of the pressure control system.

p-0036The present invention also contemplates providing an optional on-way check valve <b>68</b> downstream of restrictor <b>62</b>. Check valve <b>68</b> is configured to allow a flow gas A to the patient, but prevent a flow of gas in an opposite direction. This prevents or minimizes pressure/flow oscillations that are generated downstream of the check valve from influencing the control system upstream of the check valve.

p-0037The present invention contemplates that flow sensors <b>64</b> and <b>66</b> are any sensor suitable for use in measuring or merely monitoring the flow of gas passing through conduits. A common technique for measuring flow is to use a flow restrictor in form of a tube package or a fine-meshed net to create a pressure drop along a gas flow path. A differential pressure transducer measures the pressure drop across the flow restrictor. The signal output from the differential pressure sensor is a measure for the flow in the gas flow path. A tube package possesses good flow linearity (pressure vs. flow), but requires a rather large volume. A fine-meshed net has a poorer linearity, but constitutes itself a small volume. In order to obtain a good linearity over a large flow range, a relatively large net surface is necessitated. If the net is lying as a cross section through a circular channel, this will add to a large volume, if a good linearity is desired.

p-0038The present invention contemplates combining the components of valves <b>38</b> and <b>40</b>, flow sensors <b>64</b> and <b>66</b>, restrictor <b>62</b>, and the associated conduits, in a pneumatic block <b>100</b>, which is illustrated in <figref idrefs="DRAWINGS">FIGS. 7-9</figref>. In the illustrated exemplary embodiment, pneumatic block <b>100</b> includes three block elements <b>102</b>, <b>104</b>, and <b>106</b> that are constructed in a so-called “sandwich-technique”. A gasket <b>107</b> is provided between block elements <b>102</b> and <b>104</b>. Cavities are defined in block elements <b>102</b> and <b>104</b> to form gas flow paths <b>63</b> and <b>65</b> from valves <b>38</b> and <b>40</b> to restrictor <b>62</b>, which is also defined by or housed in the pneumatic block. A first gas source, such as gas source <b>30</b>, is coupled to a first inlet <b>108</b>, and a second gas source, such as gas source <b>32</b>, is coupled to a second inlet <b>110</b>. A first path <b>112</b> in first inlet <b>108</b> communicates the gas to first valve <b>38</b>, and a second path <b>114</b> in second inlet <b>110</b> communicates the gas to second valve <b>40</b>.

p-0039The gas flow from valves <b>38</b> and <b>40</b> is communicated to a restrictor <b>118</b>, which is defined by a pair of flow restriction nets or meshes <b>120</b> and <b>122</b>. As discussed below, restrictor <b>118</b> provides a pressure drop along the gas flow path so that the flow of gas can be measured. In the illustrated embodiment, mesh <b>120</b> and <b>122</b> are sandwiched between a bridge member <b>124</b> and a cross-bar <b>126</b> provided in gasket <b>107</b>. An advantage of restrictor <b>118</b> being in form of a large finely meshed net that is provided in one or more layers is that the volume occupied by the restrictor is kept to a minimum. The separate gas flow B and C flow through mesh <b>120</b> and <b>122</b> where they are mixed into gas flow A and output from the pneumatic block via an outlet port <b>129</b>.

p-0040Although not shown in <figref idrefs="DRAWINGS">FIG. 9</figref>, pressure sensor <b>46</b> is operatively connected to gas flow path <b>42</b> in cavity <b>128</b>, which is defined between valves <b>38</b>, <b>40</b> and restrictor <b>62</b>. Thus, cavity <b>128</b> corresponds to volume V_Bdiscussed above. As noted above, the present invention contemplates making cavity <b>128</b> larger or smaller depending on the desired amount of volume to be included in the control loop. As also noted above, the volume of cavity <b>128</b> can be made variable, for example, by providing a second chamber having a second volume that is selectively communicated to cavity <b>128</b> when it is desired to increase the volume of gas V_Bcontained between the valves and the restrictor.

p-0041A circuit board <b>130</b> is also disposed in pneumatic block <b>100</b>. Flow sensors <b>64</b> and <b>66</b>, which are differential pressure sensors, are provided on circuit board <b>130</b>. A connection terminal <b>132</b> is provided on circuit board to allow external communication with the controller <b>50</b>. The ports of the pressure sensors are connected to the upstream side of restrictor <b>62</b> via conduits <b>132</b><i>a </i>and <b>132</b><i>b</i>, and to the downstream side of the restrictor via conduit <b>134</b><i>a </i>and <b>134</b><i>b</i>. Thus, restrictor <b>118</b> provides the pressure drop needed for flow sensing, and the signals output from the differential pressure sensors is a measurement of each gas flow. Despite the fact that the restrictor is a relatively large net, and large flows will cause a non-linearity in the output signal from the flow sensors. This non-linearity may be calibrated and compensated for in the electronics unit of the measurement system, i.e., by controller <b>50</b>.

p-0042It should be understood that the present invention contemplates other configurations for restrictor <b>118</b>. For example, one flow element can be provided in gas flow path <b>63</b> to provide the pressure drop between pressure pick-off points <b>132</b><i>a </i>and <b>134</b><i>a </i>and a separate flow element can be provided in gas flow path <b>65</b> to provide the pressure drop between pressure pick-off points <b>132</b><i>b </i>and <b>134</b><i>b</i>. These flow element can have any conventional configuration. For example, a narrow portion or taper can be provided in gas flow path <b>63</b> and/or <b>65</b> to provide this pressure drop.

p-0043The present invention further contemplates providing a first temperature sensor <b>140</b> in the flow channel between valve <b>38</b> and restrictor <b>118</b>, and a second temperature sensor <b>142</b> between valve <b>40</b> and restrictor <b>118</b>. Temperature sensors <b>140</b> and <b>142</b> measure the temperature of the gases that flow out of the valves <b>38</b> and <b>40</b>. By means of this sensor signal, the gas volume may be compensated, so that it is put into relation with a desired temperature, e.g., of the ambient air, the patient's lung, or an arbitrary fixed temperature, e.g., 21° C. Temperature sensors <b>140</b> and <b>142</b> are any suitable temperature measuring device, as a thermistor, thermocouple, or an integrated silicon sensor.

p-0044As noted above, the present invention contemplates providing flow restrictor <b>62</b> downstream of cavity <b>128</b> and behind the point of measurement of pressure sensor <b>46</b>. In the illustrated embodiment, restrictor <b>62</b> is achieved as a result of the tapering of the cross sectional area of the gas flow path from chamber <b>128</b> to outlet port <b>129</b>. The present invention also contemplates that this flow restriction can be achieved by providing mesh, net, screen, or other flow restrictor in the gas flow path, either alone or in combination with the tapering of the cross-sectional area of the gas flow path. In addition, the second flow restriction enhances mixing of the first gas flow B and the second gas flow C.

p-0045As noted above, the outlet of each valve <b>38</b> and <b>40</b> has an associated flow meter <b>64</b> and <b>66</b>, with the minimum volume connected thereto. This flow meter provides input signals to the flow regulator of each valve. The outlet of the valves is subsequently connected to a small, common chamber <b>128</b>. A pressure sensor <b>46</b> is connected to this chamber, whose purpose is to provide a measurement point for the pressure in pressure control system having a relatively small volume. This embodiment provides a pneumatic block that has relatively short gas flow paths. As a result, small volumes are achieved in the pneumatic circuit even when two valves are used in combination, and, at the same time, a fast and stable control of the valves is achieved.

p-0046Pressure sensors <b>160</b> and <b>162</b> for monitoring the inlet pressures can also be provided on circuit board <b>130</b> and placed in fluid communication via conduits <b>164</b> and <b>166</b>. These inlet pressures can be used to compensate the pressure/flow characteristics of valves <b>38</b> and <b>40</b> In other words, the flow of gas output from the valves is dependent on the inlet pressure. Pressure sensors <b>160</b> and <b>162</b> monitor this inlet pressure, which can then be used to maintain the proper output from the valves.

p-0047A technique for controlling the operation of electromagnetic valves <b>38</b> and <b>40</b> will now be described with reference to <figref idrefs="DRAWINGS">FIGS. 10A-10C</figref>. It should be noted that these figures illustrate a first embodiment of a schematic circuit diagram for controlling the operation of only one of the valves. Thus, the circuit shown in <figref idrefs="DRAWINGS">FIG. 10A</figref> for example, would be provided for each of valves <b>38</b> and <b>40</b>.

p-0048In order to reduce the opening time of valves <b>38</b> and <b>40</b>, the present invention contemplates driving these valves using a pulse-width-modulated voltage that is considerably higher than the rated voltage of the valve. In this manner, a desired current through the valves is produced rapidly. Hence, a high voltage quickly raises the current through the valve to a desired value. Thereafter, the voltage is pulse width modulated, so that the current follows a desired set-point value. During the time that the voltage pulse is off, a free-wheel diode is connected across the valve. This diode allows the stored energy to circulate, with the valve current through the valve during that time. In this manner, high power efficiency is achieved. When a quick shutting of a valve is needed, the energy stored in the electromagnet of the valve has to be removed. This may be done by connecting the free-wheel diode in series with a voltage source having a reversed polarity in relation to the regular voltage pulses. This driving configuration may likewise be pulse width modulated, such that a set-point of the current is followed.

p-0049A suitable circuit <b>170</b> for the above-described method is an H-bridge, which includes two active switches S_Aand S_Band two diodes D_Aand D_Bconnected across a power supply V and a capacitor C. <figref idrefs="DRAWINGS">FIGS. 10A-10C</figref> illustrate the various operating states for the circuit. Switches S_Aand S_Boperate under the control of controller <b>50</b>.

p-0050As shown in <figref idrefs="DRAWINGS">FIG. 10A</figref>, in order to open the valve as fast as possible, switches S_Aand S_Bare closed. As a result, current flows from capacitor C, as well as from power supply V through the valve. When the desired current through the valve is reached (which is measured, for example, via current measurement resistors), switch S_Ais opened and switch S_Bis maintained close. As a result, the valve current will circulate through diode D_Aand switch S_B, as shown in <figref idrefs="DRAWINGS">FIG. 10B</figref>. The voltage across the valve U_Vwill then switch polarity and correspond to the diode voltage in the conducting diode D_Aalong with resistive voltage drops in switch S_B. Thereafter, the desired current through the valve is controlled by pulse width modulating switch S_A. When the voltage pulses via S_Aare not being applied to the H-bridge, diode D_Awill work as a free-wheel diode.

p-0051If it is desired to close the valve rapidly, i.e., if too high of a current still flows through the valve, switch S_Bshould also be opened. As shown in <figref idrefs="DRAWINGS">FIG. 10C</figref>, when switch S_Aand S_Bare opened, diodes D_Aand D_Bare placed in series with the reversed voltage U_V, corresponding to the supply voltage V. As a result, energy is removed from the valve much faster, and this energy is fed back into capacitor C and the power supply line.

p-0052The present invention also contemplates that diodes D_Aand D_Bmay be replaced by electrically controlled switches S_Dand S_E. This alternative embodiment is illustrated in <figref idrefs="DRAWINGS">FIG. 11</figref>. The control signals to these switches have in this case to be inverted in relation to the control of the switches in series in the H-bridge.

p-0053The principle of the invention is more advantageous because the energy of the actuating magnet is directly and harmonically transferred to the power supply capacitor C. Thus, the losses that appear (50%), in the case when a capacitor is discharged into another capacitor, are avoided, and furthermore the surge currents, which occur during such a discharge process, are avoided, which leads to an improved reliability, less interferences and above all a longer service life of the power supply capacitor.

p-0054The following table 1 summarize the different modes of the above-described control of the valves.

p-0055<tables id="TABLE-US-00001" num="00001"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="7"><colspec colname="1" colwidth="28pt" align="left" /><colspec colname="2" colwidth="28pt" align="center" /><colspec colname="3" colwidth="42pt" align="left" /><colspec colname="4" colwidth="28pt" align="center" /><colspec colname="5" colwidth="42pt" align="center" /><colspec colname="6" colwidth="28pt" align="center" /><colspec colname="7" colwidth="21pt" align="left" /><thead><row><entry namest="1" nameend="7" align="center" rowsep="1" /></row><row><entry>Mode</entry><entry>S_A</entry><entry>D_A</entry><entry>S_B</entry><entry>D_B</entry><entry>C</entry><entry>FIG.</entry></row><row><entry namest="1" nameend="7" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry>Fast</entry><entry>closed</entry><entry>currentless</entry><entry>closed</entry><entry>currentless</entry><entry>dis-</entry><entry>10A</entry></row><row><entry>opening</entry><entry /><entry /><entry /><entry /><entry>charges</entry></row><row><entry>Control</entry><entry>closed</entry><entry>currentless</entry><entry>closed</entry><entry>currentless</entry><entry>dis-</entry><entry>10A</entry></row><row><entry>(pulse)</entry><entry /><entry /><entry /><entry /><entry>charges</entry></row><row><entry>Control</entry><entry>open</entry><entry>conducting</entry><entry>closed</entry><entry>currentless</entry><entry>un-</entry><entry>10B</entry></row><row><entry>(free-</entry><entry /><entry /><entry /><entry /><entry>affected</entry></row><row><entry>wheel)</entry></row><row><entry>Rapid</entry><entry>open</entry><entry>conducting</entry><entry>open</entry><entry>conducting</entry><entry>charges</entry><entry>10C</entry></row><row><entry>Shutting</entry></row><row><entry>Shutting</entry><entry>open</entry><entry>conducting</entry><entry>closed</entry><entry>currentless</entry><entry>un-</entry><entry>10B</entry></row><row><entry>(free-</entry><entry /><entry /><entry /><entry /><entry>affected</entry></row><row><entry>wheel)</entry></row><row><entry namest="1" nameend="7" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

p-0056While a valve is described above as being the means for controlling the pressure, flow, and/or volume of gas delivered to the user, it is to be understood that the present invention also contemplates controlling the pressure, flow, or volume of gas by controlling the operating speed of the pressure generator. This can be done alone or in combination with a valve. That is, in one embodiment, the operating speed of a compressor, such as the speed of a blower, alone is used to control the pressure of the patient and the valves are eliminated entirely. In another embodiment, pressure generator operating and one or more valves are used in combination to control the pressure of gas delivered to the patient.

p-0057It can be appreciated from the foregoing description and the accompanying figures that present invention provides a device for providing pressure control in the flow of gas delivered to a patient, that is capable of rapidly responding to changes in the system. This is made possible in an exemplary embodiment by providing valves operating as flow/pressure regulators and a flow restriction positioned between the valves and the outlet of the device. The volumes and dimensions of gas channels prior to the flow restriction are small with reference to the overall system, including the airway of the patient.

p-0058Although the invention has been described in detail for the purpose of illustration based on what is currently considered to be the most practical and preferred embodiments, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.